1. Field of the Invention
This invention relates to a machine tool for precisely processing a work piece into a non-circular cross-sectional configuration by inputting simplified data of such configuration.
2. Prior Art
In a machine tool for processing a work piece into a non-circular cross-sectional configuration, a conventional copying method using a pattern has a disadvantage of having to manufacture the pattern. Accordingly, such a method has recently been investigated that program data of a non-circular cross-sectional contour are worked out on a computer, by inputting the data of the contour, thereby processing the work piece-through numerical control.
In the case of processing a work piece into a non-circular cross-sectional configuration, when the work piece or a tool is displaced in the direction (Z direction) parallel to the rotation axis 0, a conventional publicly-known feed screw method and so on can fully cope with the processing of the work piece because it is not necessary to displace the work piece or the tool at any particularly high speed and further a certain speed for displacement can be fixed.
However, recently, since the speeded-up processing of a work piece necessitates rotation at very high speed, it becomes necessary to move the tool not only at high speed but also highly accurately in the X axis direction, that is to say, in the direction perpendicular to the rotation axis 0.
To the above-described need, in Japanese Patent Appln. Laid-Open No. 61-8202, for example, the working fluid of an oil hydraulic cylinder is controlled to reciprocate a tool holder, under the control of a hydraulic servo mechanism. In this method, because the stroke of a single oil hydraulic cylinder becomes so large that a problem arises in the response of the servo system due to inertial force of the moving parts, compressibility of the working fluid and the like, the control system is divided into a main and a sub parts: the main part controls the average diameter of the work piece requiring large movement by means of the conventional feed screw method, and the sub part controls the difference between the minimum and maximum diameters of each cross section of the work piece by means of the oil hydraulic cylinder. Thus, the reciprocative movement of the hydraulic cylinder must be controlled at the center of its stroke, in order to process the work piece more precisely. Accordingly, there has been a disadvantage in that the size of the cylinder tends to be restricted. Further, an instance employing an oil hydraulic servo mechanism is disclosed in Japanese Patent Applh. Laid-Open No. 61-8203.
Such mechanism employing an oil hydraulic servo mechanism has a possibility that, in accordance with an increase in the frequency of the reciprocative movement of the cylinder in the X direction by speeding up the rotation of the work piece, a delay in the phase will occur and the amplitude will decrease. Further, since the oil hydraulic servo mechanism is operated at high speed, the temperature of the working fluid rises and the characteristics change, which results in a deterioration in processing precision because of heat displacement. Further, as the working fluid is compressive, it is influenced by a difference in cutting amount and resistance during cutting caused by the rotational speed of the work piece, which is a primary factor in causing a deterioration in processing precision.
There are other factors which cause a possibility that a delay in the phase will occur and the amplitude will decrease, following an increase in the frequency of the reciprocative movement of the cylinder in the X direction caused by speeding up the rotation of the work piece. Among these are the weight of the tool holder provided in a conventional machine tool, that is, of the slider portion, and a change in the cutting resistance and so on.
There is disclosed in U.S. Pat. No. 4,653,360, an instance employing a linear motor mechanism in place of the above-described oil hydraulic servo mechanism. However, in this instance, because the tool is directly reciprocated in the X direction by the linear motor, the one linear motor bears loads such as the whole weight of the tool holder and the change in the cutting resistance. Thus, this instance has a problem causing the precision during cutting at high speed to deteriorate.
Further, since a cooling device for suppressing production of heat from the linear motor is required, the above instance is not economical.
In addition, the conventional machine tool for processing a work piece into a non-circular cross-sectional configuration has the disadvantage that the data to be input amounting to several tens of thousands of numbers, is difficult to use easily, as in the above publicly-known instance.